A so-called field device is designed so as to detect a physical quantity such as pressure, temperature, flow, or the like, in various kinds of plants, to convert the detected value into an electric signal, to transmit the electric signal to upper rank apparatus through a transmission path or, conversely to receive control signals transmitted from the upper rank apparatus to thereby control valves, and so on, of the plant. As for the transmission of electric signals, they are standardized if they are analog signals, and analog current signals of 4 to 20 mA are transmitted between the field devices and the upper rank apparatus. Generally, one-way communication with analog signals is performed between field devices and upper rank apparatus.
Recently, as the technique of semiconductor integrated circuits has advanced, a field device including a microprocessor has been developed and realized. According to this, not only the above-mentioned one-way communication with analog signals, but also bidirectional communication with digital signals can be performed on a transmission path, so that the establishment of the range of the field device, the self-diagnosis thereof, and so on, can be instructed by remote control.
In addition, there has been recently proposed a field bus system in which a plurality of field devices are connected on one and the same transmission path by means of a multi-drop system so as to realize communication with only bidirectional digital signals.
In this field bus system, a plurality of field devices and upper rank apparatus are connected to form a tree structure through a transmission path. The plurality of field devices are operated through the transmission path by electric power supplied from an external power source so as to perform bidirectional communication with the upper rank apparatus on the transmission path by use of digital signals sequentially, and perform the transmission of detected physical quantity, the reception of a control value, and so on.
Upper communication apparatus are connected between the field devices and the upper rank apparatus or the external power source so as to perform bidirectional communication with the field devices, and so on, by use of digital signals. In addition, terminators respectively constituted by a resistor and a capacitor connected in series are connected on either side of the transmission path respectively.
In the case where an existing system of analog signals is replaced by such a field bus system, it is indeed necessary to change upper rank apparatus and field devices to those corresponding to field bus, but a transmission path can be used as it is. Accordingly, it is possible to shift the system easily, and it is possible to increase the number of field devices connected on the transmission path. It is therefore considered that the system can be extended easily.
However, when an existing system of analog signals is replaced by such a field bus system, a plurality of field devices are connected to a single transmission path, and digital communication is performed thereon. Accordingly, it is necessary to take the reliability of antinoise into consideration in comparison with the existing system. Further, a problem exists in that the control of field devices at a constant period cannot be performed if communication failure occurs frequently.
The field bus system is used for measurement and control operation in various kinds of plants. In the field bus system, the physical quantity in the plant is detected at intervals of a predetermined period (for example, 0.5 seconds), and the control instruction is operated on the basis of the detected physical quantity. The operated control instruction is supplied to a field device controlling a valve, or the like, at intervals of the predetermined period. If the physical quantity is not detected at the predetermined period, the field device cannot be controlled correctly
Therefore, the physical quantity must be detected at the predetermined intervals correctly, and the control instruction must be supplied to the field device at the predetermined intervals correctly. In the field bus system, the communication between the field devices and the upper rank apparatus, or the like, is performed, and the detection of the physical quantity is performed at the predetermined intervals.
Therefore, if the communication failure occurs frequently, the physical quantity cannot be detected correctly at the predetermined intervals, and consequently, the field device cannot be controlled correctly.
In the case of a field bus system, a transmission path has characteristics that the transmission distance is so long that the attenuation, rounding and distortion of a transmission signal are very large depending on conditions in comparison with a usual communication network. In addition, such a field bus system is installed in a field so that noise environment is strict. Taking the above points into consideration, communication signals of respective devices are set to large values in a range of from 0.75 to 1.0 V.sub.p-p. In the case of field devices, operation power is supplied through the transmission path, so that communication signals are electric current signals in a range of from 15 to 20 mA.sub.p-p.
In view of the above communication signals, it has therefore been necessary to set the electric current consumption of the field device to a value of 8 mA or more. That is, in order to superimpose communication signals in a range of .+-.7.5 to 10 mA.sub.p-p onto a DC operating current, it is necessary to set the value of the DC operating current to 8 mA or more. Therefore the number of field devices which can be connected to the field bus, under conditions which are intrinsically safe and timeproof, with limited energy, i.e., a limited power supply, has been restricted to a small number.
In a signal transmission method disclosed in JP-A 5-41709, therefore, in a field bus system, a DC current is set to a low level when no communication signal is transmitted, while the DC current level is increased at the time of communication signal transmission so as to reduce the electric current consumption per field device, and hence increase the number of field devices which can be connected to the field bus system.
That is, for example, the DC current level is set to 4 mA when no communication signal is transmitted, while the DC current level is increased to 8 mA at the time of communication signal transmission. Since it is considered to be a rare case when a plurality of field devices transmit signals at the same time, the electric current consumption of the field devices is reduced, and the number of the field devices which can be connected to the field bus system can therefore be increased.
However, in the signal transmission method disclosed in the above-mentioned JP-A 5-41709, the voltage of a transmission path becomes oscillatory when the DC current level is changed from a low level to a high level, for example, from 4 mA to 8 mA, so that the reliability of communication signals is reduced. Therefore, in the signal transmission method according to the above-mentioned publication, the DC current level is increased from 4 mA to 8 mA gradually, and decreased from 8 mA to 4 mA gradually.
Therefore, in the signal transmission method according to the above-mentioned publication, not only is it necessary to provide means for increasing the DC current level at the time of transmission, and for decreasing the DC current level at the time of non-transmission, but it is also necessary to provide means for relaxing the increase and decrease of the DC current level. Accordingly, the structure is complicated.